Image forming system

ABSTRACT

An image recording apparatus for recording an image on a sheet by a main scanning with a laser beam and a sub-scanning of the sheet, the image recording apparatus having: a pair of upstream rollers, arranged upstream of an exposing position of the laser beam; a pair of downstream rollers, wherein the sub-scanning is conducted by nipping and conveying the sheet with the pairs of upstream rollers and downstream rollers; wherein the pair of downstream rollers includes: a first roller driven by a drive section; a second roller, having a total weight of 300 g or less, wherein a gap amount of a nip between the first and the second rollers is set smaller than a thickness of the sheet; and a roller holding section for holding both ends of the second roller so as to be capable of driven rotation.

BACKGROUND OF THE INVENTION

The present invention relates to an image recording apparatus forrecording an image wherein a sheet-like object to be scanned is conveyedwhile being sandwiched between a pair of rollers.

Patent Document 1 discloses a method for recording an image on the wholesurface of a sheet as a recording medium by conveying it with two setsof nip rollers. It is well known that an image defect (irregularitiesacross the sheet feed direction) tends to be produced by the flapping ofthe sheet or irregular rotation of the drive system, when the leadingedge of the sheet enters the second nip roller located downstream andthe tailing edge of the sheet gets out of the upstream first nip roller,according to this method. To solve the problem caused when the tailingedge of the sheet gets out of the upstream first nip roller, it is acommon practice to release the first nip roller before the sheet getsout of the roll, as described in the Patent Document 2.

To solve the problem caused when the leading edge of the sheet entersthe second nip roller, a proposal has been made of a method of detachingthe nip when the sheet enters the second nip roller, as described in thefollowing Patent Document 3. According to this method, irregularitiescaused by entry of the film can be prevented. However, when the detachedroller contacts the sheet, irregularities still tend to occur. To removesuch irregularities, a proposal has been made of a method of keeping theroller rotating before it comes in contact with the sheet (PatentDocument 3). However, this method involves a complicated mechanism forrotating and rocking the roller (concurrent revolving and rotating). Andbecause of using heavy weight and large inertia rollers, this method isnot sufficient from the point of reducing the irregularities at the timeof contact. The timing of occurrence of the irregularities has merelyshifted in the direction of conveyance.

The applicants of the present invention have proposed a method asdescribed in the following Patent Document 4. According to this method,a pair of rollers is detached from each other with a predetermineddistance prior to sheet entry to reduce the irregularities at the timeof sheet entry. This method does not depend on the aforementioned themechanism of concurrent revolution and rotation. Further, in theJapanese Patent Application TOKUGAN-2003-025863, the present applicantsproposed a method wherein the nip pressure of the second roller can beswitched in such a way that a decreased nip pressure is selected at thetime of sheet entry and an increased nip pressure is selected subsequentto sheet entry, thereby reducing the irregularities at the time of sheetentry and ensuring stable sheet conveyance.

Incidentally, the laser imager (image recording apparatus) for medicaluse is required to produce a mammographic image output of a multiplesize (8×10-inch sized through 14×17-inch sized recording medium). Forthe purpose of interpreting a printed film includingmicro-calcification, the mammographic image is required to providehigher image quality and stability than that of normal modality. Thisrequires further reduction of the rotational irregularities of the drivesystem, and it is necessary to avoid the flapping of the film producedwhen the sheet enters the nip roller and gets out of it or variations inload due to irregular rotation of the drive system.

With downsizing of the apparatus, the curvature of the conveyance pathafter the second nip roller will be increased. The exposure section willbe affected by the degree of the toughness (rigidity) of the filmlocated at the position corresponding to this curvature, in response tothe film size. Thus, the conveyance force of the nip roller will lose toan increased resistance of conveyance by the guide, with the result thata slip will occur during conveyance. There is concern about thepossibility of such problems.

The prior art pressure contact roller (nip roller) comprises a 10mm-diameter through-shaft made of a stainless steel or the like, forkeeping rotation, capable of conveying a 14-inch wide film; and a 20mm-diameter roller portion made of rubber, metal material or the likedirectly in contact with the film. The weight of the total nip rollerassembly is about 500 through 600 g. Thus, according to the method ofusing a pair of rollers detached from each other with a predetermineddistance prior to sheet entry, as described in the following PatentDocument 4, the second pair of rollers must be rotated prior to filmentry; otherwise, there will be increased irregularities at the time offilm entry. Further, if foreign substances such as dust and dirt havedeposited on the structural portion (hitting runner portion) for thispreliminary rotation, the hitting runner will become loose by thethickness of foreign substances. When coming in contact with the film ordrive roller again, the potential energy is released by the quantitycorresponding to thickness of foreign substances, and disturbance willbe applied to the film and drive system at each rotation of the hittingrunner. This will result in irregularities appearing in the image.Further, the outer diameter of the roller is changed with the passage oftime due to the difference in the materials of the conveyance sectionand hitting runner and in the coefficients of linear expansion andvolatilization of the additives of the rubber roller for hardnessadjustment. This will result in unstable conveyance. There is concernabout the possibility of such problems.

[Patent Document 1] Official Gazette of Japanese Patent Tokkaisho62-94068

[Patent Document 2] Official Gazette of Japanese Patent Tokkaihei02-264563

[Patent Document 3] Official Gazette of Japanese Patent Tokkaisho62-135064

[Patent Document 4] Official Gazette of Japanese Patent Tokkaihei09-156797

SUMMARY OF THE INVENTION

In view of the prior art problems described above, it is an object ofthe present invention to provide an image recording apparatus capable ofavoiding an adverse effect caused by the entry of foreign substances onthe roller pair to convey the sheet-like object for sub-scanningtransportation, whereby a high-quality image is achieved.

One aspect of the present invention for the solution of theaforementioned problems is concerned with an image recording apparatusfor recording an image on a sheet-like object to be scanned, wherein theaforementioned sheet-like object is subjected to sub-scanning whilebeing conveyed, sandwiched by a pair of upstream rollers arrangedupstream from the position exposed to a laser beam and a pair ofdownstream rollers arranged downstream from that position, and isconcurrently subjected to main scanning by application of the laserbeam; the aforementioned pair of upstream rollers comprising:

-   -   a first roller driven by a drive section;    -   a second roller, having a total weight of 300 g or less, capable        of carrying the sheet-like object to be scanned, by sandwiching        it in collaboration with the first roller, wherein a nip gap        with the first roller is set at a level smaller than that of the        sheet-like object to be scanned; and    -   a holding section for holding both ends of the second roller in        such a manner as to be driven.

Another aspect of the present invention is concerned with an imagerecording apparatus for recording an image on a sheet-like object to bescanned, wherein the aforementioned sheet-like object is subjected tosub-scanning while being conveyed, sandwiched by a pair of upstreamrollers arranged upstream from the position exposed to a laser beam anda pair of downstream rollers arranged downstream from that position, andis concurrently subjected to main scanning by application of the laserbeam; the aforementioned pair of upstream rollers comprising:

-   -   a first roller driven by a drive section;    -   a second roller, having a starting torque of 1.7 gf·cm or less,        capable of carrying the sheet-like object to be scanned, by        sandwiching it in collaboration with the first roller, wherein a        nip gap with the first roller is set at a level smaller than        that of the sheet-like object to be scanned; and    -   a holding section for holding both ends of the second roller in        such a manner as to be driven.

According to the aforementioned image recording apparatus, in theaforementioned downstream roller pairs to convey the recording mediumfor sub-scanning, the second roller has a smaller weight and/or lowerstarting torque than that of the prior art, and such holding means as abearing is configured so as to be driven. Accordingly, when a sheet-likeobject to be scanned has entered the nip gap, the second roller iseasily driven and the nip gap expands rapidly according to thethickness. This arrangement reduces the irregularities when the leadingedge of the sheet-like object to be scanned has been fed by the upstreamroller pair. At the same time, this arrangement eliminates the need ofpreliminary rotation of the second roller. This makes it possible toeliminate the use of the prior art runner having a poor resistance toincoming foreign substances, and to avoid a bad effect of the incomingforeign substances upon the downstream roller, whereby a high-qualityimage can be obtained.

The aforementioned image recording apparatus is provided with anadjusting section for adjusting the nip gap, and the adjusting sectionhas an energizing section working on the gravity component of the secondroller working on the first roller side, in the direction opposite tothat of gravity component. This energizing section allows the nip gap tobe adjusted. Thus, the required gap can be obtained by simple procedureof adjusting the position of the energizing section (adjustment ofenergizing force).

It is preferred to include a nip pressure controller for changing thenip pressure between the first and second roller.

In this case, it is preferred that control be provided to increase thenip pressure a predetermined time after the leading edge of thesheet-like object to be scanned has entered the downstream roller. Thisincreases the nip pressure of the downstream roller pair a predeterminedtime after the leading edge of the sheet-like object to be scanned hasentered the downstream roller. This arrangement reduces theirregularities at the time of entry of the sheet-like object, andimproves the conveying performance after the entry.

It is preferred that the nip pressure controller include a rotarysolenoid, and the rotary solenoid be gradually moved under the pulsewidth modulation control.

The image recording apparatus of the present invention eliminates anadverse effect of the entry of foreign substances such as dust and dirt,upon the downstream roller for conveying the sheet-like object forsub-scanning, whereby a high-quality image is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of the major portions of an image recordingapparatus as an embodiment of the present invention;

FIG. 2 is a schematic drawing of the optical beam scanner of the imagerecording apparatus of FIG. 1;

FIG. 3 is a perspective view showing the sub-scanning mechanism of anoptical beam scanner 120 of FIGS. 1 and 2;

FIG. 4 is a side view of the major portions showing a first conveyanceroller pair 170 of FIG. 3;

FIG. 5 is a side view of the major portions showing a second conveyanceroller pair 180 of FIG. 3;

FIG. 6 is a front view schematically showing the second conveyanceroller pair 180 of FIGS. 3 and 5;

FIG. 7 is a block diagram showing the control system of the sub-scanningmechanism of the optical beam scanner 120 of FIGS. 1 through 6;

FIG. 8 is a diagram showing the relationship between the weight of thedriven roller of a second conveyance roller pair in the first embodimentand evaluation of image quality based on irregularities at the time ofentry of the leading edge; and

FIG. 9 is a diagram showing the relationship between the starting torqueof the driven roller of a second conveyance roller pair in the secondembodiment and evaluation of image quality based on irregularities atthe time of entry of the leading edge.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following describes the best form of the embodiment of the presentinvention with reference to drawings: FIG. 1 is a front view of themajor portions of an image recording apparatus as an embodiment of thepresent invention. FIG. 2 is a schematic drawing of the optical beamscanner of the image recording apparatus of FIG. 1.

As shown in FIG. 1, the image recording apparatus 100 comprises: a feedsection 110 having first and second loading sections 11 and 12 forloading a package of a predetermined number of films as sheet-likethermal development photosensitive materials (recording media), and asupply section 90 for conveying and supplying each of the films forexposure and development; an optical beam scanner 120 for forming alatent image by scanning two-dimensionally while sub-scanning, andexposes the film fed from the feed section 110; a developing section 130for thermally developing the film with the latent image formed thereon;a densitometer 200 for measuring the density of the developed film andgetting the density information; a cooling conveyance section 150 forconcurrently cooling and conveying the heated film; and an ejection tray160 where the film is ejected.

Each sheet of film is conveyed from the first and, second loadingsections 11 and 12 of the feed section 110 by means of the supplysection 90 and a pair of conveyance rollers 139 and 141 in thearrow-marked direction (1). By controlling each of the supply sections90, it is possible to switch and convey the sheets of film from thefirst and second loading sections 11 and 12.

Referring to FIG. 2, the following describes the optical beam scanner120 of the image recording apparatus 100. As shown in FIG. 2, in theoptical beam scanner 120, the rotary polygon mirror 113 deflects thelaser beam L having a predetermined wavelength within the range from 780through 860 nm with its intensity having been modified based on theimage signal S of the diagnostic image information inputted into theinput section 133. The optical beam scanner 120 scans the surface of thefilm F, and at the same time, provides sub-scanning with causingrelative movement of the film F in the horizontal directionapproximately at right angles to the main scanning direction withrespect to the laser beam L by the sub-scanning mechanism composed ofrollers 171 and 181. The optical beam scanner 120 uses the laser beam Lto form the latent image on the film F. It should be noted that eachdriven roller of the first and second conveyance roller pairs 170 and180 is not illustrated in FIG. 2.

In the optical beam scanner 120 of FIG. 2, when the image signal S as adigital signal inputted from the outside is inputted through the inputsection. 133, the image signal S is converted into the analog signal andis inputted into the modulator 123. Based on this analog signal, themodulator 123 controls the driver 124 of the laser beam source 110 a, toemit the laser beam L modulated from the laser beam source 110 a.

After having passed through the lens 112, the laser beam L emitted fromthe laser beam source 110 a is converged only in the vertical directionby the cylindrical lens 115. In this case, the laser beam L enters tothe polygon mirror 113 as a linear image perpendicular to the drive axisof the rotary polygon mirror 113 rotating in the arrow-marked directionA of FIG. 2. The rotary polygon mirror 113 reflects and deflects thelaser beam L in the main scanning direction, and the deflected laserbeam L passes through the fθ lens 114 including the cylindrical lensconsisting of a combination of four lenses. After that, the laser beam Lis reflected by the mirror 116 arranged in extension on the optical pathin the main scanning direction, and provides main scanning repeatedly inthe arrow-marked direction X on the scanned surface 117 of the film Fwhich is being fed (sub-scanned) in the arrow-marked direction Y by thefirst and second conveyance roller pairs 170 and 180. This procedureallows the laser beam L to scan the entire surface of the scannedsurface 117 on the film F.

The cylindrical lens of the fθ lens 114 allows the incoming laser beam Lto be converged on the surface of the film F to be scanned only in thesub-scanning direction. The distance from the fθ lens 114 to the surfaceof the film F to be scanned is equal to the focal distance of the entirefθ lens 114. In this manner, the cylindrical lens 115 and fθ lens 114including a cylindrical lens are arranged in the optical beam scanner120. The laser beam L is once converged on the rotary polygon mirror 113only in the sub-scanning direction. Accordingly, even if planarinclination or shaft vibration has occurred to the rotary polygon mirror113, a scanning line of regular pitch can be formed, without thescanning position of the laser beam L deviating in the sub-scanningdirection on the surface of the film F to be scanned. The rotary polygonmirror 113 has a better scanning stability than a galvanometer or otherlight deflectors. In this manner, a latent image in conformity to theimage signal S is formed on the film F.

The following describes the developing section 130 of the imagerecording apparatus given in FIG. 1. As shown in FIG. 1, the developingsection 130 comprises: a heating drum 14 capable of heating the film Fwhile holding it on its outer periphery; and a plurality of opposingrollers 16, arranged face to face with the heating drum 14, for holdingthe film by sandwiching it with the heating drum 14. The film is heatedwhile being sandwiched between the heating drum 14 and opposing rollers16, and is conveyed by the rotation of the heating drum 14.

The heater (not illustrated) arranged inside the heating drum 14 isplaced under power on/off control and maintains the temperature at alevel equal to or greater than a predetermined thermal developmenttemperature (for example, about 110 degrees Celsius) for a predeterminedduration of time, whereby the film F is heated and subjected to thermaldevelopment. Thus, the latent image formed on the film F by the opticalbeam scanner 120 is formed into a visible image. Further, density rangecan be adjusted by changing the heater temperature by the power supplycontrol.

The densitometer 200 of FIG. 1 contains a light emitting section 200 aand a light receiving section 200 b. When the developed film is conveyedbetween the light emitting section 200 a and light receiving section 200b in the manner described above, the light emitted from the lightemitting section 200 a is received by the light receiving section 200 bthrough the film, and the density is measured in conformity to thedegree of the attenuation in the amount of received light. Based on themeasured density information, the amount of the laser beam of theoptical beam scanner 120 is fed back and controlled so that the filmfinishing accuracy will be constant.

The film conveyed in the arrow-marked direction (1) is then conveyed inthe arrow-marked direction (2) and a latent image is formed as describedabove. Then the film is fed to the heating drum 14 of the developingsection 130 in the arrow-marked direction (3) by the conveyance rollerpair 142 and is subjected to thermal development and a latent image isvisualized, as described above. After that, while being cooled, the filmis conveyed by the conveyance roller pair 144a and 144, and its densityis measured by the densitometer 200. The film is further fed in thearrow-marked direction (4) by the conveyance roller pair 144 and isejected to the ejection tray 160.

Referring to FIGS. 3 through 7, the following describes the sub-scanningmechanism of the optical beam scanner 120.

FIG. 3 is a perspective view showing the sub-scanning mechanism of anoptical beam scanner 120 of FIGS. 1 and 2. FIG. 4 is a side view of themajor portions showing a first conveyance roller pair 170 of FIG. 3.FIG. 5 is a side view of the major portions-showing a second conveyanceroller pair 180 of FIG. 3. FIG. 6 is a front view schematically showingthe second conveyance roller pair 180 of FIGS. 3 and 5. FIG. 7 is ablock diagram showing the control system of the sub-scanning mechanismof the optical beam scanner 120 of FIGS. 1 through 6.

As shown in FIGS. 1 and 3, the sub-scanning mechanism of the opticalbeam scanner 120 includes a first conveyance roller pair 170 arrangedupstream and a second conveyance roller pair 180 arranged downstream.While laser beam L emitted from the laser beam source 110 a isirradiated between the first conveyance roller pair 170 and secondconveyance roller pair 180 as shown in FIG. 2, the film is conveyed bythe conveyance roller pairs 170 and 180.

As shown in FIGS. 1 through 6, the first conveyance roller pair 170located upstream in the sub-scanning direction Y contains adriving-roller 171 driven by a motor (not illustrated) in the directionof rotation R′ given in FIG. 4, and a driven roller 172 driven by thedriving roller 171. The second conveyance roller pair 180 locateddownstream in the sub-scanning direction Y contains a driving roller 181driven by a motor (not illustrated) in the direction of rotation R givenin FIG. 4, and a driven roller 182 driven by the driving roller 181.

As shown in FIG. 6, in the second conveyance roller pair 180, the drivenroller 182 has two roller sections 182 a, of the same diameter on bothsides with respect to the driving roller 181, and has a plurality ofroller sections 182 b having the same diameter as those of the rollersections 182 a at the intermediate position. In the driven roller 182, athrough-shaft 182 c is held rotatably by the bearings 182 d and 182 e onboth ends on the further outer side of the roller sections 182 a, sothat the driven roller 182 can be driven.

To ensure that a high friction coefficient will be applied to thesurface in contact with the film, the roller sections 182 a and 182 b ofthe second conveyance roller pair 180 are made of rubber materials suchas silicone rubber and EPDM, and are fitted or bonded onto the outerperiphery of the through-shaft 182 c. The through-shaft 182 c is made ofsuch light metal as aluminum or its alloy. The second conveyance rollerpair 180 has a total mass of 300 gf or less, more preferably 200 gf orless, as a roller assembly. The driving roller 181 is made of steel suchas stainless steel.

The second conveyance roller pair 180 can be moved integrally with thebearings 182 d and 182 e, with respect to the driving roller 181. Aswill be described later, a nip gap 183 is formed between the drivingroller 181 and a plurality of roller sections 182 a and 182 b of thedriven roller 182 in such a way that of the nip gap 183 does not exceedthe thickness of the film.

Referring to FIGS. 3, 4, 5 and 7, the following describes the mechanismfor switching the nip pressure of the first conveyance roller pair 170and second conveyance roller pair 180 and the control system thereof.

As shown in FIGS. 3 and 4, the sub-scanning mechanism of the opticalbeam scanner 120 comprises: a first driving source 50 consisting of arotary solenoid for switching the nip pressure of the first conveyanceroller pair 170; a shaft 51 moved by the first driving source 50 in thehorizontal direction m of FIG. 4 and in the opposite horizontaldirection m′; a shaft member 52, connected to the shaft 51, extending inthe same direction as that of the first conveyance roller pair 170 andsecond conveyance roller pair 180 (direction approximately orthogonal tothe horizontal directions m and m′); a first lever member 54 forrotating the driven roller 172 relative to the drive roller 171 byrotating about the fulcrum shaft 6b in the rotating direction s andopposite rotating direction s′, with the movement of the shaft member 52in the horizontal directions m and m′; and a biasing member 53, havingits one end fixed to the securing section 100 a on the casing side ofthe apparatus, for applying energy so as to drive the first lever member54 in the rotating direction s′.

Further, as shown in FIGS. 3 and 5, the sub-scanning mechanism of theoptical beam scanner 120 comprises: a second drive source 56 consistingof a rotary solenoid for switching the nip pressure of the secondconveyance roller pair 180; a shaft 57 moved by the second drive source56 in the horizontal direction n of FIG. 5 and in the oppositehorizontal direction n′; a shaft 57 a connected with the shaft 57through the biasing member 58 so as to be energized in the horizontaldirection n; a shaft member 63, connected to the shaft 57 a, extendingin the same direction as that of the first conveyance roller pair 170and second conveyance roller pair 180; a second level member 59 forrotating the driven roller 182 relative to the drive roller 181 byrotating about the fulcrum shaft 61 in the rotating direction t andopposite rotating direction t′, with the movement of the shaft member 63in the horizontal directions n and n′; and a biasing member 64, havingits one end fixed to the securing section 100 b on the casing side ofthe apparatus, for applying energy so as to drive the second levelmember 59 in the rotating direction t′. It should be noted that therotary solenoid (second drive source 56) in FIG. 3 is not illustrated.

In the second conveyance roller pair 180 of FIG. 5 the driven roller 182works on the side of the drive roller 181 under the gravity. However,the biasing member 64 works in the direction opposite to the driveroller 181 against the gravity of the driven roller 182, with the resultthat the driven roller 182 is energized in the direction of moving awayfrom the drive roller 181 against the gravity (direction of decreasingthe nip pressure). Then the balance of energizing force between thesecond drive source 56 and biasing member 58 is adjusted. Thus, when therotary solenoid of the second drive source 56 is located at the neutralposition, the nip gap 183 is formed between the drive roller 181 and aplurality of roller sections 182 a and 182 b of the driven roller 182,as shown in FIG. 6, in such a way that the thickness of the nip gap 183does not exceed the film thickness.

The control system of the image recording apparatus 100 given in FIG. 7contains: a reflection type optical sensor 149 arranged upstream fromthe first conveyance roller pair 170 given in FIG. 1; and a controller148 including a central processor unit (CPU) for controlling the firstdriving source 50 and second drive source 56 given in FIGS. 4 and 5,based on the detection signal from the optical sensor 149, and switchingthe nip pressure between the first conveyance roller pair 170 and secondconveyance roller pair 180.

A beam is applied to the film from the optical sensor 149 and thereflected beam thereof enters the light receiving section of the opticalsensor 149, whereby the leading edge of the film fed by the conveyanceroller pair 141 given in FIG. 1 is detected by the optical sensor 149and a detection signal is issued. Based on the detection signal comingfrom the optical sensor 149, the controller 148 determines the timedintervals and controls the driving sources 50 and 56 so that the nippressure is switched. Control is provided in such a way that the nippressure is increased after a predetermined period of time from theleading edge of the film has entered the second conveyance roller pair180.

The controller 148 of FIG. 7 controls the rotary solenoids of the firstdriving source 50 and second drive source 56 through pulse widthmodulation. To be more specific, the controller 148 allows the phaseangle of the rotary solenoid to be changed by adjusting the on/off dutyratio. When the duty ratio is gradually changed and the phase angle isgradually adjusted, the nip pressures of the first conveyance rollerpair 170 and second conveyance roller pair 180 can be gradually changedand switched, independently of each other.

The following describes the operation of the sub-scanning mechanism ofthe optical beam scanner in the image recording apparatus given in FIGS.1 through 7. In the first place, when the film is fed in the direction(1) given in FIG. 1 from the first and second loading sections 11 and 12by the supply section 90 and the conveyance roller pairs 139 and 141,the leading edge of the film is detected by the optical sensor 149 ofFIG. 1, and the leading edge of the film enters the first conveyanceroller pair 170. The controller 148 controls the first driving source 50and rotates the rotary solenoid of the first driving source 50 of FIG. 4so that the phase angle of the rotary shaft 50 a will change. When theshaft 51 has moved in the horizontal direction m, the first lever member54 is pushed through the shaft member 52, whereby the driven roller 172is driven in the rotating direction s of FIG. 4. This procedureincreases the nip pressure with respect to the drive roller 171 of thedriven roller 172 in the first conveyance roller pair 170, and allowsthe first conveyance roller pair 170 to feed the film stably in thesub-scanning direction Y.

In the meantime, the nip gap 183 having a thickness smaller than that ofthe film shown in FIG. 6 is formed before the film enters the secondconveyance roller pair 180. If the film has entered this nip gap 183,the driven roller 182 rotates easily since the driven roller 182 is heldrotatably by the bearings 182 d and 182 e of FIG. 6. Further, lightweight permits easy movement in such a way that the nip gap 183 willincrease. This arrangement reduces obstacles when the leading edge ofthe film enters the nip gap 183 of FIG. 6. The driven roller 182 is setby the bearings 182 d and 182 e not to exceed the starting torque of 1.7gf#cm is held so as to rotate easily when the film has entered. Thisarrangement eliminates the need of previously rotating the driven roller182.

After the film has entered the second conveyance roller pair 180, therotary solenoid of the first driving source 50 rotates the rotary shaft50 a under the control of the controller 148, and moves the shaft 51 inthe horizontal direction m′ of FIG. 4. Then the first lever member 54 ispushed in the horizontal direction m′ by the energizing force of thebiasing member 53, and the driven roller 172 is driven in the rotatingdirection s′, thereby reducing the nip pressure in the first conveyanceroller pair 170 and releasing the pressure. This procedure reducesobstacles when the trailing edge of the film gets out of the firstconveyance roller pair 170.

In the meantime, after a predetermined period of time from the film hasentered the second conveyance roller pair 180, the controller 148controls the second drive source 56 on the second conveyance roller pair180, and drives the rotary solenoid of the second drive source 56 ofFIG. 5 so that the phase angle of the rotary shaft 56 a will change.When the shaft 57 has been moved in the horizontal direction n, thesecond level member 59 is driven in the rotating direction t against theenergizing force of the biasing member 64, through the biasing member58, shaft 57 a and shaft member 63, whereby force is applied in thedirection where the driven roller 182 moves toward the drive roller 181(in the direction of decreasing the nip gap 183). Accordingly, in thesecond conveyance roller pair 180, the nip pressure with respect to thedrive roller 181 of the driven roller 182 is increased. This procedureallows the second conveyance roller pair 180 to feed the film stably andimproves film conveying performances.

In each of the aforementioned operations, the rotary shaft 50 a androtary shaft 56 a of the rotary solenoid rotate gradually, instead ofrapidly, and the rotation is accompanied by a small change in phaseangle. The shaft 51 and shaft 57 gradually move in the directions m, n,m′ and n′. The nip pressures in the first conveyance roller pair 170 andsecond conveyance roller pair 180 change gradually, instead of rapidly.This arrangement ensures smooth switching of each nip pressure in thefirst conveyance roller pair 170 and second conveyance roller pair 180,and reduces vibration, without giving an adverse effect to the filmconveyance for sub-scanning.

As described above, the sub-scanning mechanism of the optical beamscanner given in FIGS. 1 through 7 prevents a conveyance trouble fromoccurring and eliminates a bad effect upon the image formation duringthe conveyance of the film in the sub-scanning direction Y, when theleading edge of the film enters the first and second conveyance rollerpairs 170 and the film gets out of the first conveyance roller pair 170.

Further, a predetermined period of time after the film has entered thesecond conveyance roller pair 180, the nip pressure of the secondconveyance roller pair 180 is switched over to the higher pressure sothat the conveyance power is increased. This ensures stable conveyanceof the film in the sub-scanning direction, and hence improved imagequality.

As described above, the present embodiment pays attention to the secondconveyance roller pair 180 located downstream, in particular, to thestructure and weight. It solves the problem of irregularities at thetime of entry of the film and ensures stable film conveyance, byreducing the starting torque for rotation of the driven roller 182 andreducing the weight, through reviewing the structure and the weight,wherein the prior art of applying the hitting runner is not used.

To be more specific, the prior art conveyance roller pair as rollerassemblies has a weight of 500 through 600 gf. This requires preliminaryrotation prior to film entry. If foreign substances such as dust anddirt have deposited on the structural portion for preliminary rotation(hitting runner), then disturbance will be applied to the film and drivesystem at each rotation of the hitting runner. This will result inirregularities appearing in the image. By contrast, the presentembodiment uses bearings to provide low starting torque, free rotationand light weight of the driven roller 182. This arrangement eliminatesthe need of preliminary rotation at the time of film entry and adoptionof hitting runners, thereby solving the problem of irregularities at thetime of entry of the film leading edge, and preventing possible adverseeffect of the deposition of foreign substances such as dust and dirtupon image quality.

In FIGS. 5 and 6, the amount of gap of the nip gap 183 prior to theentry of the film can be set at an appropriate level by adjusting thebalance in the energizing forces of the biasing member 58 and biasingmember 64. If the amount of gap of the nip gap 183 is adjusted, it ispossible to determine the nip pressure in the time duration from theentry of the film to the operation of the pressure controller by thecontroller 148 and second drive source 56.

Embodiment 1

The following gives more specific description of the present inventionwith reference to the first embodiment. The weight of the driven roller182 of the second conveyance roller pair 180 in FIGS. 3 and 5 waschanged to 100, 200, 300, 400 and 540 gf in that order, and the springconstant was changed, thereby adjusting the balance of the energizingforce between the biasing member 58 and the biasing member 64. The gapamount of the nip gap 183 of FIG. 6 was changed to 100, 125, 150 and 190μm in that order. Then the image was recorded and the disturbed image(irregularities in the image) at the time of entry of the film leadingedge were observed, thereby evaluating the image quality. FIG. 8 showsthe result of this experiment. In the image quality evaluation in FIG.8, the image is not affected by irregularities at the time of filmentry, if the evaluation score is “Excellent” (almost no disturbed imageat the time of film entry) or better.

In the first embodiment, the driven roller 182 used is constructed insuch a way that the through-shaft 182 c is made of aluminum (cylindricalform) having a diameter of 16 mm, and silicone rubber having a thicknessof 2 mm (JIS A hardness 70 through 75 deg.) is coated on a predeterminedportion, where the outer diameter of the rubber layer is 20 mm. Thedrive roller 181 used is made of a stainless steel having a diameter of20 mm. Model MF 106ZZS by NSK Ltd. is used as the bearings 182 d and 182e of driven roller 182, and the film thickness is 200 μm.

FIG. 8 shows that the image quality is deteriorated by theirregularities at the time of entry of the film leading edge moreseriously as the weight of the driven roller 182 is greater and the gapamount is smaller. When the gap amount is 100 through 190 μm, the imagequality is hardly affected by irregularities at the time of film entryif the weight of the driven roller 182 does not exceed 300 gf, and theimage quality is not affected by irregularities at the time of filmentry if the weight of the driven roller 182 does not exceed 200 gf.

Emvodiment 2

The following gives more specific description of the present inventionwith reference to the second embodiment. The weight of the driven roller182 of the second conveyance roller pair 180 in FIGS. 3 and 5 waschanged to 100, 200, 300, 400 and 540 gf in that order, and the springconstant was changed, thereby adjusting the balance of the energizingforce between the biasing member 58 and biasing member 64. The gapamount of the nip gap 183 of FIG. 6 was changed to 100, 125, 150 and 190μm in that order. Then the image was recorded and the disturbed image(irregularity in the image) at the time of entry of the film leadingedge were observed, thereby evaluating the image quality. FIG. 9 showsthe result of this experiment. In the image quality evaluation in FIG.9, the image is not affected by irregularities at the time of filmentry, if the evaluation score is “Excellent” (almost no disturbed imageat the time of film entry) or better.

In the second embodiment, the driven roller 182 used is constructed insuch a way that the through-shaft 182 c is made of aluminum (cylindricalform) having a diameter of 16 mm, and silicone rubber having a thicknessof 2 mm (JIS A hardness 70 through 75 deg.) is coated on a predeterminedportion, where the outer diameter of the rubber layer is 20 mm. Thedrive roller 181 used is made of a stainless steel having a diameter of20 mm. The driven roller 182 used has a weight of 200 gf, whereinfriction coefficient between the roller and film emulsion surface is2.0. The film thickness is 200 μm.

Model MF106ZZSPS2-S, MF106ZZSPS-L, MF1060T12MC3 D4MA and MF106T12ZZ1MC3NS7A by NSK Ltd. is used as the bearings 182 d and 182 e of drivenroller 182. The starting torque of the driven roller 182 was changed forevaluation. FIG. 9 shows that the image quality is deteriorated by theirregularities at the time of entry of the film leading edge moreseriously as the starting torque of the driven roller 182 is greater andthe amount of gap is smaller. When the gap amount is 100 through 190 μm,the image quality is hardly affected by irregularities at the time offilm entry if the starting torque of the driven roller 182 does notexceed 1.7 gf·cm, and the image quality is not affected byirregularities at the time of film entry if the weight of the drivenroller 182 does not exceed 1.3 gf·cm.

The present invention has been described with reference to theembodiments and examples. It is to be expressly understood, however,that the present invention is not restricted thereto. The presentinvention can be embodied in a great number of variation, withoutdeparting from the technological spirit of the present invention. Forexample, timing of increasing the nip pressure of the second conveyanceroller pair 180 can be set appropriately in conformity to the positionof the guide member 190 downstream from the second conveyance rollerpair 180 and the radius dimensions thereof.

1. An image recording apparatus for recording an image on a sheet by conducting a main scanning of the sheet with a laser beam while conducting a sub-scanning of the sheet, the image recording apparatus comprising: a pair of upstream rollers, arranged upstream of an exposing position of the laser beam; a pair of downstream rollers, arranged downstream of the exposing position of the laser beam, wherein the sub-scanning is conducted by nipping and conveying the sheet with the pair of upstream rollers and the pair of downstream rollers; wherein the pair of downstream rollers comprises: a first roller driven by a drive section; a second roller, having a total weight of 300 g or less, capable of carrying the sheet by nipping the sheet in cooperation with the first roller, wherein a gap amount of a nip between the first roller and the second roller is set smaller than a thickness of the sheet; and a roller holding section for holding both ends of the second roller in such a manner as to be capable of driven rotation.
 2. The image recording apparatus of claim 1, further comprising an adjuster for adjusting the gap amount of the nip, the adjuster comprising a biasing section for biasing to an opposite direction of a gravity component of the second roller, the gravity component working on the first roller side, wherein adjuster is capable to adjust the gap amount of the nip by using the biasing section.
 3. The image recording apparatus of claim 1, further comprising a nip pressure controller for changing a nip pressure between the first roller and the second roller.
 4. The image recording apparatus of claim 3, wherein the nip pressure controller controls to increase the nip pressure a predetermined time after the leading edge of the sheet has entered in the nip of the pair of downstream rollers.
 5. The image recording apparatus of claim 3, wherein the nip pressure controller comprises a rotary solenoid, and the rotary solenoid is gradually moved under a pulse width modulation control.
 6. An image recording apparatus for recording an image on a sheet by conducting a main scanning of the sheet with a laser beam while conducting a sub-scanning of the sheet, the image recording apparatus comprising: a pair of upstream rollers, arranged upstream of an exposing position of the laser beam; a pair of downstream rollers, arranged downstream of the exposing position of the laser beam, wherein the sub-scanning is conducted by nipping and conveying the sheet with the pair of upstream rollers and the pair of downstream rollers; wherein the pair of downstream rollers comprises: a first roller driven by a drive section; a second roller, having a starting torque of 1.7 gf·cm or less, capable of carrying the sheet by nipping the sheet in cooperation with the first roller, wherein a gap amount of a nip between the first roller and the second roller is set smaller than a thickness of the sheet; and a roller holding section for holding both ends of the second roller in such a manner as to be capable of driven rotation.
 7. The image recording apparatus of claim 6, further comprising an adjuster for adjusting the gap amount of the nip, the adjuster comprising a biasing section for biasing to an opposite direction of gravity component of the second roller, the gravity component working on the first roller side, wherein the adjuster is capable to adjust the gap amount of the nip by using the biasing section.
 8. The image recording apparatus of claim 6, further comprising a nip pressure controller for changing a nip pressure between the first roller and the second roller.
 9. The image recording apparatus of claim 8, wherein the nip pressure controller controls to increase the nip pressure a predetermined time after the leading edge of the sheet has entered in the nip of the pair of downstream rollers.
 10. The image recording apparatus of claim 8, wherein the nip pressure controller comprises a rotary solenoid, and the rotary solenoid is gradually moved under a pulse width modulation control. 